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Wave power

Posted on 20 September 2012

Wave power

Any swimmer that gets drawn too far away from the coastline knows the power of the ocean. The strength of each wave and the continual undulation of ocean water can make it a formidable place. The raw power of the ocean also makes it an exciting potential resource for renewable energy researchers.

Next month, the first ocean wave energy-capturing device with a permit to send energy onto a U.S. public power grid will be deployed in the Pacific Ocean. The device is called the PowerBuoy and was designed by the New Jersey-based company Ocean Power Technologies. The company envisions the buoy to be the first in a “wave park” two and a half miles off the shore of Reedsport, Ore. Eventually Ocean Power Technologies would like to place 11 PowerBuoys in the wave park to capture energy from the ocean and send it back to shore via a cable buried in the seafloor.

An Ocean Power Technology PowerBuoy lays on its side before entering the ocean. The buoy bobs vertically when deployed and only the yellow top is visible above the waves. The bottom end is anchored to the sea floor and a cable runs from the generator in the middle of the buoy all the way back to shore to transmit power to land. Photo: Sylvia Duckworth

How much power do ocean waves hold?

Ocean Power Technologies is one of the leaders in developing devices to capture the ocean’s energy according to researcher Paul Jacobson. Jacobson is the water power program manager for the Electric Power Research Institute (EPRI), a non-profit organization that conducts research on electric power generation. EPRI recently assessed all U.S. coastline for its potential as a source of wave power. They found that Alaska and the Pacific northwest have the densest, most powerful wave action.

Every year, Americans use 4 trillion kilowatt-hours of electricity. How much of that could be supplied by ocean waves? According to Jacobson, three numbers answer that question. Each year U.S. coastline waves conduct 2.6 trillion kilowatt-hours of energy. But even though that equals more than half of the amount of energy Americans use, the amount that we could capture is smaller. Jacobson labels the fraction of total wave energy humans could harness the “technically capturable” portion. Some waves are too big and some are too small so devices like the PowerBuoy can’t absorb all of the ocean’s energy. The “technically capturable” portion of the ocean’s total wave energy is closer 1.1 trillion kilowatt-hours, or about 25 percent of U.S. energy needs.

Before using that number to dream of fueling a quarter of the country’s power with ocean waves, there’s still one more distinction Jacobson makes. The “technically capturable” number doesn’t subtract areas where ships travel, environmentally sensitive locations, or places where population is so sparse it is inefficient to gather energy from the ocean. The realistic percentage of U.S. power that could be supplied by wave energy is less than the “technically capturable” 25 percent. Director of the Northwest National Marine Renewable Energy Center (NNMREC) Belinda Batten says that many researchers say the ocean could provide approximately 10 percent of U.S. power with its waves.

WPD is Wave Power Density and kW/m is kilowatts per meter.

EPRI’s map shows the varying concentration of wave power along U.S. and Puerto Rican coastline. Most wave power is concentrated in Alaska and the Pacific northwest. Image: National Renewable Energy Laboratory

NNMREC is a collaboration between Oregon State University and the University of Washington. At NNMREC, Oregon State University (OSU) focuses on wave energy while the University of Washington develops another type of ocean energy, tidal. Batten is professor of mechanical engineering at OSU and helps maintain NNMREC’s testing area for new wave energy power devices. The testing area is a swath of ocean in which designers can see how their ideas function in real life. Capturing wave energy is such a new idea that standard technology doesn’t really exist for it yet. There are lots of different ideas about how to absorb the ocean’s power and turn it into electricity. NNMREC’s testing area is where the rubber meets the road, or rather, the buoys meet the waves.

The ups and downs of wave energy

As a form of renewable energy, waves have several benefits over solar and wind power. Batten says that the predictability of waves is a big one. Knowing when clouds will cover the sun or when the wind will die off is difficult. Wave behavior is easier to predict. “You can forecast waves 84 hours in advance,” says Batten. “It’s really pretty impressive.”

Another big plus for wave energy is that it is on the coasts, where a lot of the U.S. population lives. The closer the energy source is to the people who use it, the better, according to Jacobson, because it is more efficient to transport energy 10 miles than 1000 miles.

The most challenging part about capturing energy from ocean waves is that they are located, well, in the ocean. “It’s a harsh environment with high energy,” says Jacobson. Developers want to place their devices in the middle of the most powerful waves, but really powerful waves make it difficult to both deliver and do maintenance work on the device. “High energy sites are attractive but extreme waves and corrosive water are real challenges,” says Jacobson.

What about ocean-dwellers?

As this new field of renewable energy develops, environmental groups and researchers alike are interested in what will happen to the organisms already inhabiting the oceans. When a big PowerBuoy moves in, is it a friendly neighbor? Jacobson says two main concerns center on the mooring lines that anchor the devices to the ocean floor and the electromagnetic field the power cables emit. The mooring lines are a potential hazard for marine life because there is a risk or getting tangled in them.

Gray whales are one of the many, many organisms with whom the wave energy capturing devices will share the ocean. The gray whale migrates each year and environmental groups want to make sure that energy devices do not disrupt the whale’s migratory path. Photo: Jeff Huffman

Risks from the electromagnetic field (EMF) emitted by cables transporting power back to land is a little more complicated. In laboratory tests where fish were directly exposed to strong electromagnetic fields, the fish stopped moving and the field changed the function of their gills. But EPRI has done a lot of EMF research and Jacobson says so far the results, though showing there could be an effect on fish, aren’t good at simulating what real conditions would be like if a device were present in the ocean. There aren’t many opportunities to monitor how actual devices would affect actual oceanic life since the technology is so new.

Additionally, environmental groups like Oregon Shores Conservation Coalition want to make sure locations for wave energy-capturing devices are carefully chosen. They say it is important to avoid sensitive habitats and not interfere with migratory paths.

Government permitting for ocean wave energy devices depends in part on data showing the environmental effects of proposed devices. Jacobson pointed out that without actual devices to monitor, that data is hard to produce. “It’s the chicken or the egg problem,” says Jacobson. “Questions about environmental effects can best be assessed by monitoring operating projects, but in order for there to be an operating project, the permitting needs to be completed.”

Time will tell if such projects can harvest significant amounts of energy and co-exist with marine life, but the need for humans to find new and renewable sources of energy are likely to bring more projects like this on line.